Chromatography is a method of chemical analysis in which chemical components in a mixture are separated from each other before detection. The fundamental mechanism of separation is based on the relative distributions of the mixture components between a mobile phase and a stationary phase. The most popular form of chromatography is where the stationary phase is packed or coated inside a length of tubing, and the mobile phase flows through the tubing (or chromatographic column) by the application of pressure. The stationary phase can be an adsorbent or an immobilized polymeric material, while the mobile phase can be a gas, a liquid, or a supercritical fluid. The mobile phase carries the sample through the chromatigraphic column where the separation takes place and into a detector for detection of the separated components.
Detectors can be classified as either universal (nonselective) or selective. Universal detectors give a response for most chemicals, while selective detectors only respond to chemicals that contain certain elements or structural features. For example, the flame ionization detector is classified as a universal detector because it responds to any organic carboncontaining compound, and the flame photometric detector can be made specific for only sulfur-containing compounds by using an appropriate filter. Universal detectors are used when one is interested in the total composition of the sample mixture, while selective detectors are used to detect certain compounds or classes of compounds. Selective detectors are generally much more sensitive than universal detectors.
Among the most important selective detectors in chromatography are the element selective detectors. The most popular element-selective detectors are the electron capture detector (ECD), thermionic ionization detector (TID), the flame photometric detector (FPD), and the electrolytic conductivity detector (ELCD).
The ECD can achieve sub-picogram detection of halogen (F, Cl, Br) containing compounds, and has been widely used for the detection of pesticides, insecticides, drugs, and environmental pollutants. However, response factors and, hence, sensitivity vary considerably from compound to compound depending on the environment of the halogen atom in the molecule. The TID allows the most sensitive detection for compounds containing nitrogen and phosphorus, and has been used in the analysis of pesticides, petroleum, fossil fuels, food and flavors, clinical samples, drugs, and so on.
The FPD is mainly used for the selective detection of compounds containing sulfur or phosphorus, but can be used for the detection of halogens, nitrogen, and various metals. This detector is not as sensitive as the other element selective detectors, and the sensitivity can be further reduced by quenching of the signal if water or hydrocarbons are coeluted with the sample. Furthermore, the sulfer response of the FPD can vary from first-order to second-order depending on the heteroatom environment. The FPD has been widely used for the analysis of pollutants in air and water, pesticides, and fossil fuel-derived materials.
The ELCD offers picogram detection of nitrogen and sulfer, and sub-picogram chlorine detection limits. It is an important detection tool in the monitoring of environmental pollutants. However, due to the difficulties in its routine operation, column-to-detector interfacing, and loss of sensitivity due to the use of a solvent purge for minimizing connector, reactor, and scrubber dead volume, the ELCD is not as widely used as the other element-selective detectors.
In order to circumvent many of the problems associated with the element-selective detectors described above, and to produce a detector that can be tuned to any of a large number of elements instead of only a few, electrical discharge-type detectors have been investigated.
Several types of electrical discharges have been used for element selective detectors in chromatography, but all have suffered from deficiencies that have prevented their widespread use. Inductively coupled argon plasmas effectively decompose large amounts of organic materials, but provide poor excitation efficiency for the non-metallic elements that are of primary chromatographic interest. Inductively coupled plasmas have been operated with helium to give improved excitation of nonmetals, but the instrumentation is complex and expensive to operate. Microwave plasmas efficiently excite nonmetallic elements in low concentrations, but are intolerant of large sample loads and are often extinguished by the passage of the solvent during a chromatographic run. Direct current discharges have inhomogeneous excitation characteristics and tend to be unstable because of electrode heating and erosion. Nitrogen afterglow discharges produce complex background spectra and poor excitation of some nonmetals.
Gay, U.S. Pat. No. 4,509,855, discloses a direct current atmospheric pressure helium plasma emission spectrometer which contains two side-arms that receive effluents from a micro-column liquid chromatograph and a gas chromatograph. The plasma region is viewed through a light pipe positioned at an opening in the discharge tube. The opening penetrates directly into the plasma region leaving the plasma exposed to the atmosphere. The plasma is initiated when sufficient voltage is applied across the electrodes, and a large damping resistor is used to permit automatic reignition of the plasma after it is extinguished by the passage of a large quantity of sample.
Dodge, U.S. Pat. No. 4,309,187, discloses a method of analyzing for trace amounts of metals and other species capable of excitation by energy transfer from metastable excited nitrogen molecules. The metastable nitrogen is produced in a dielectric discharge and excites the species to be analyzed. The nitrogen source used for excitation is disclosed as air used at pressures below atmospheric pressure. The complex background spectrum produced by nitrogen discharges precludes their use as selective detectors for nonmetallic elements (Rice et al., Anal. Chem., 1981 53, 1519-1522).
Hagen, U.S. Pat. 4,532,219, discloses a microwave induced plasma used to create characteristic spectra from molecules and atoms immobilized in the discharge. Uniquely, Hagen used an apparatus that allows for the introduction of a non-volatile sample into the plasma cavity, and the plasma is transported to the sample following sample introduction.
Rice, U.S. Pat. No. 4,586,368, discloses a radio frequency electrodeless discharge. A grounding pin is used above the primary discharge electrode to produce an intense afterglow from which elemental emission is observed during a chromatographic run. In the visible and ultraviolet regions, where most emissions are monitored, interferences from molecular bands cause the selectivity of the detector to be poor.